Gas removal method and composition

ABSTRACT

A SELECTIVE METHOD OF REMOVING SULFUR DIOXIDE FROM FLUE GASES OR THE LIKE CONTAINING BOTH SULFU DIOXIDE AND HYDROGEN SULFIDE WITHOUT REMOVING THE HYDROGEN SULFIDE BY BUBBLING THE FLUE GAS THROUGH ABOUT A 5-95 PERCENT SOLUTION OF AN ORGANIC ACID AND AN ALKALI METAL SALT OF THE SAID ACID. THE ACID IS SELECTED FROM THE GROUP OF CITRIC, MALIC, LACTIC, SUCCINIC, ASCORBIC AND TARTARIC. THE ACID IS PREFERABLY CITRIC AND THE ALKALI METAL IS PREFERABLY POTASSIUM. THE RATIO OF THE WEIGHT OF THE SALT TO THAT OF THE ACID MAY BE WITHIN THE RANGE OF ABOUT 1S:2A TO 6S:1A OR SUCH AS IS NEEDED TO MAKE THE SOLUTION PH BETWEEN ABOUT 4.0 TO 6.0, WHERE S AND A ARE ONE MOLE EACH OF SAID SALT AND SAID ACID, RESPECTIVELY. THE SAID RATIO PREFERABLY IS 5S:1A OR THE PH IS PREFERABLY 5.7.

United States Patent 3,833,508 GAS REMOVAL METHOD AND COMPOSITION Robert R. Austin, Pasadena, and Arthur Leonard Vincent, Monrovia, CallL, assignors to International Telephone and Telegraph Corporation, New York, N.Y.

No Drawing. Continuation-impart of application Ser. No. 137,994, Apr. 27, 1971, now Patent No. 3,757,488. This application Oct. 12, 1971, Ser. No. 188,540

Int. Cl. C01]: 31/16,- Ctllk 3/00 US. Cl. 252189 5 Claims ABSTRACT OF THE DISCLOSURE A selective method of removing sulfur dioxide from flue gases or the like containing both sulfur dioxide and hydrogen sulfide without removing the hydrogen sulfide by bubbling the flue gas through about a 5-95 percent solution of an organic acid and an alkali metal salt of the said acid. The acid is selected from the group of: citric, malic, lactic, succinic, ascorbic and tartaric. The acid is preferably citric and the alkali metal is preferably potassium. The ratio of the weight of the salt to that of the acid may be within the range of about lsz2a to 6s:1a or such as is needed to make the solution pH between about 4.0 and 6.0, where s and a are one mole each of said salt and said acid, respectively. The said ratio preferably is 5s: In or the pH is preferably 5.7.

BACKGROUND OF THE INVENTION This application is a continuation-in-part of copending application Ser. No. 137,994, filed Apr. 27, 1971, by the inventors hereof for Gas Removal Method and Composition, now US. Pat. No. 3,757,488. The benefit of the filing date of said copending application is therefore hereby claimed for this application.

This invention relates to the art of separating gases, and more particularly, to a method of separating sulfur dioxide from hydrogen sulfide and other gases, and a composition which may be employed in practicing the method.

The method and composition of the present invention will be found useful in many applications and are, therefore, not to be limited to those disclosed herein. However, the present invention as been found especially useful in separating and monitoring sulfur dioxide and/or hydrogen sulfide in line gases.

In the paper industry, it is of interest especially in the recent past to monitor sulfur dioxide and hydrogen sulfide in flue gases. Monitoring is required for the protection of the ecology. That is, the extent and cause of a pollution problem must be known before a solution can be found.

Such monitoring has already been accomplished by alternately passing the sample gas: (1) directly through a coulometric titrator as disclosed in US. Pat. No. 3,448,- 031; and (2) first through a 3 percent solution of potassium biphthalate (KI-10 11 0,) and then through the coulometric titrator. The biphthalate solution, as taught in the prior art, will remove about 95 percent of the SO, while passing most of the H S. The output of the coulometric titrator is an electrical signal whose amplitude is directly proportional, on the direct flow sample, to the sum of the sulfur dioxide and the hydrogen sulfide in the flue gas and on the filtered sample, to the approximate concentration of hydrogen sulfide in the flue gas. The S0 content is obtained by subtracting the titration current for the H 8 reading from the titration current from the H,S+SO reading.

Notwithstanding the desirability of monitoring the sulfur dioxide and hydrogen sulfide independently, the abovedescribed prior art system suffers from three serious disadvantages.

Patented Sept. 3, 1974 The first is that the prior art solution has a very short life. For example, the prior art solution, when having a concentration, by weight, of about 2 percent, will remove about percent of the sulfur dioxide for only a period of about 6 hours. The sulfur dioxide absorption of the prior art solution is also undesirably low, and hydrogen sulfide rejection is not as good as is desired.

SUMMARY OF THE INVENTION In accordance with the present invention, the abovedescribed and other disadvantages of the prior art are overcome by the use of one of a group of organic acids with an alkali metal salt thereof.

In accordance with the invention, an aqueous solution of a 23 percent concentration by weight of solid will have a life of about 8 days during which sulfur dioxide absorption will be 99.8 percent. Further, the solution of the invention has a better hydrogen sulfide rejection than the said prior art solution.

The above-described and other advantages of the present invention will be better understood from the following description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the present invention may be practiced by contacting flue gases containing both sulfur dioxide and hydrogen sulfide with an aqueous solution of an organic acid and an alkali metal salt of the self same acid. The acid should be selected from the group of: citric acid, malic acid, lactic acid, succinic acid, ascorbic acid and tartaric acid.

Contacting may be performed by passing the flue gas through a conventional scrubber containing the said solution. For example, it is common to use such a scrubber incorporating an air tight receptacle in which the solution is contained, the receptacle having a tubular gas outlet sealed thereto having an opening inside the receptacle above the uppermost level of the solution. A tubular gas inlet is also sealed thereto. The gas inlet has a lower end which extends downwardly inside the receptacle and is located so as to release the flue gas below the said uppermost level of the said solution so as to bubble the flue gas through the solution. Bubblers of several different varieties may be employed, if desired. For example, the bubbler disclosed in the said patent may be employed in accordance with the present invention.

In accordance with the foregoing, both sulfur dioxide and hydrogen sulfide contained in the main flow of line gases may be independently monitored. The apparatus for performing the method of the present invention to monitor either one or both of the sulfur dioxide and hydrogen sulfide gases may be entirely conventional apart from the said solution and, if desired, may be of the prior art type disclosed or referred to herein.

The said solution of the present invention may be made in several ways. In all cases, the concentration is preferably in excess of 5 percent. It also may be more convenient to make up a solution with a concentration of about 95 percent or less, or at least a solution which does not contain the acid and salt in excess of saturation. How- ?vler, a more concentrated solution will have a longer useu life.

The ratio of the weight of the salt to the acid may be between about 1s:2a and 6s:1a, where s and a are one mole each of the said salt and the said acid, respectively. Preferably, the said ratio is about 5s: la.

The ratio may also be made for a desirably pH range. It was erroneously calculated to be 4.0 to 5.6, but was later found to be 4.0 to 6.0 by use of a pH meter. The preferred pH was erroneously calculated to be 5.4 but was later found to be 5.7 by use of the pH meter.

According to a preferred embodiment of the present invention, the acid is citric acid and the salt is potassium citrate.

The phrase percent of concentration, when used without the phrase by weight is hereby defined for use herein and in the claims to mean the percent of saturation at 20 C. of the least soluble component of the acid-salt system. Note will be taken that, for example regardless of the salt mixed with water, the resulting component may or may not be the self same salt. Thus potassium citrate may reduce to potassium dihydrogen citrate or potassium monohydrogen citrate.

The word component" is, therefore, hereby defined as including anything but water in the said solution, including, but not limited to, the acid and salt of the original mix and compounds or elements resulting from any changes thereof produced in the solution.

EXAMPLE NO. 1

An aqueous solution of 0.2 mole of citric acid and 1.0 mole potassium citrate in one liter of water was made. This resulted in a solution concentration of 23 percent, by weight. The pH of the resulting solution was erroneously calculated to be 5.4 but was later found to be 5.7 on the pH meter. Sulfur dioxide was bubbled through this solution. The 99.8 percent of the sulfur dioxide was absorbed from a sample gas at 300 parts per million SO; concentration for a period of about 8 days.

EXAMPLE NO. 2

A solution of citric acid of a concentration of 50 percent was prepared. Enough potassium hydroxide was added to the said citric acid solution to increase pH of the resulting solution to what was erroneously calculated to be 5.4. However, the pH was found to be 5.7 by the use of the pH meter. S: was bubbled through this solution and the solution absorbed 99.8 percent of the S0 EXAMPLE NO. 3

A solution was prepared by mixing 1.2 moles (161 grams) of malic acid with 2.4 moles (97 grams) of sodium hydroxide in a liter of water. This solution had a pH of 5.7. S0, was bubbled through this solution. The absorption of the S0, was 99.8 percent.

EXAMPLE NO. 4

A solution was prepared by mixing 1.0 mole of potassium citrate with 0.2 mole of citric acid in a liter of water. Enough citric acid was further mixed with the solution to decrease its pH to 4.1. S0 was then bubbled through the solution. This resulted in a 80; absorption of 93 percent.

4 EXAMPLE NO. 5

A solution was prepared by mixing 1.0 mole of potassium citrate with 0.2 mole of citric acid in a liter of water. Enough potassium hydroxide was then mixed with the solution to increase its pH to 6.0. H 8 absorption was then 6 percent.

From the foregoing it will be appreciated that the inventive solution absorption of S0; is quite large at a pH of 5.7 and higher and is still 93 percent at a pH of 4.0. Further, there is substantially complete H S rejection at and below a pH of 5.7 and a 94 percent rejection at 6.0. The low end of the preferred pH range is thus determined by the minimum percent S0 absorption permitted. The high end of the preferred pH range is determined by the minimum H 8 rejection permitted.

What is claimed is:

1. A scrubber solution for use in the removal of sulfur dioxide from a mixture including both sulfur dioxide and hydrogen sulfide without removing the hydrogen sulfide from the mixture, said scrubber solution comprising: an organic acid; at least one alkali metal salt of said acid, said acid being a material selected from the group consisting of citric acid, malic acid, lactic acid, succinic acid, ascorbic acid and tartaric acid; and a quantity of water, said acid and said salt being in and forming a solution with said water, said formed solution having a concentration in excess of five percent.

2. The invention as defined in Claim 1, wherein the ratio of the weight of said alkali salt of said acid to that of said acid is such that the pH of said solution is between about 4.0 and 6.0.

3. The invention as defined in Claim 1, wherein the pH of said solution is about 5.7.

4. The invention as defined in Claim 3, wherein said solution has a concentration of about 95 percent.

5. The invention as defined in Claim 4, wherein said acid is citric acid, said salt being potassium citrate.

U.S. Dispensatory, p. 334 (1960). Chemical Abstracts, vol. 36, col. 2591. Mercks Index, 7th Edition (1966) pp. 266 and 839.

I OHN D. WELSH, Primary Examiner us. Cl. X.R. 

